Examples of typical ground improving methods include a jet grouting method and a chemical churning piling method. According to the jet grouting method, a ground area is dug and grouted using a super high pressure jet water and an air surrounding the jet water. A ground injecting agent is injected and filled into a space dug and grouted by the jet water and air. This method has advantages such that the arrival distance of the jet water is extended and the ground can be grouted over a wide range (large section area). However, since the grouted slimes are blown up onto the ground due to the lifting operation of the air, a problem arises because the grouted slimes, blow up onto the ground, should be properly treated. Also, if cement milk is used as a ground improving agent, a problem arises such that part of the cement milk is blown up onto the ground and a secondary environmental pollution is caused due to the alkalinity of cement.
On the other hand, in the chemical churning piling method, the ground improving agent is injected into the ground at a high pressure, the ground is dug and grouted by the destructive power thereof, and a consolidation body of the ground improving agent is constructed in the ground. Since no air is used, no grouted slime is blown up onto the ground. Thus, the problem of treatment of the grouted slimes etc. does not occur. However, a drawback occurs because the arrival distance of the ground improving agent is relatively short and the ground cannot be grouted over a wide range.
The applicant has already proposed a ground improving method in which a ground improving agent (such as cement milk, cement bacillus, or the like), injected at a high pressure, is surrounded by a liquid (such as water) which is injected at a lower pressure. Frictional resistance between the ground improving agent and the sediments is reduced, thereby increasing the arrival distance of the ground improving agent.
The above-mentioned method itself is effective. However, in the case of using only an amount of jet stream which is the same as an amount of ground improving agent necessary for grouting, a problem arises because the amount of jet stream will be less than a necessary injection amount of the improved consolidation body of a large area. In addition, it is generally necessary to immediately throw away grouted slimes after the ground improving agent has been used. The costs for throwing away these slimes are expensive.
Examples of conventional techniques have been proposed in U.S. Pat. No. 4,084,648, entitled "PROCESS FOR THE HIGHPRESSURE GROUTING WITHIN THE EARTH AND APPARATUS ADAPTED FOR CARRYING OUT SAME", and U.S. Pat. No. 4,047,580, entitled "HIGH-VELOCITY JET DIGGING METHOD".